It is well known that a non-conductive tubular polyimide film is generally processed into a belt form to be used, for example, as a belt for conveying heated materials and a fixing belt for use in an electrophotographic system.
Moreover, a semi-conductive tubular polyimide film comprising conductive carbon black mixed and dispersed in a non-conductive tubular polyimide film is used as an intermediate transfer belt for use in, for example, copiers, printers, facsimiles, and presses.
It is known that such non-conductive and semi-conductive tubular polyimide films are prepared by forming a predetermined film-formation starting material into a flat film, and jointing both ends of the flat film to form a tubular shape; or by forming a predetermined film-formation starting material into a seamless tubular film by centrifugal casting in a single step. Japanese Unexamined Patent Publication No. 2000-263568 filed by the applicant of the present application discloses performing centrifugal casting under substantially no centrifugal force to form a tubular shape.
In general, a solution of polyamide acid (or polyamic acid) with high-molecular-weight (a number average molecular weight: usually about 10000 to about 30000), i.e., a polymer precursor of a polyimide, is used as a film-formation starting material for such tubular polyimide films.
More specifically, the polyamic acid solution is produced by polycondensation of aromatic tetracarboxylic dianhydride and an equimolar amount of aromatic diamine in an organic polar solvent at low temperatures at which no imidization occurs. Examples of the aromatic tetracarboxylic dianhydride include 1,2,4,5-benzene tetracarboxylic dianhydride, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, 3,3′,4,4′-benzophenone tetracarboxylic dianhydride, 2,3,6,7-naphthalene tetracarboxylic dianhydride, etc., in which the anhydride groups are point-symmetrically arranged. Examples of aromatic diamines include p-phenylenediamine, 4,4′-diaminodiphenylether, 4,4′-diaminodiphenylmethane, etc.
The method for producing a polyimide film ordinarily comprises three steps: preparing a polyamic acid solution as a film-formation starting material; forming this into a polyamic acid film; and imidizing the formed film.
However, a polyamic acid solution obtained by the preparation steps described above is disadvantageous in that a partial gel is likely to be gradually formed during storage due to the pot life thereof. Such partial formation of gel is likely to occur at high temperatures, but it proceeds with time even at low temperatures. Thus, even when the gel formation is negligible, it is a matter of course that the physical properties of the finished polyimide film prepared from such a solution are adversely affected and, in addition, the flatness of the film is diminished. In particular, the addition of conductive carbon black to the polyimide film leads to an increase in the variation of electrical resistance.
Moreover, since the solubility of polyamic acid resins in an organic polar solvent is limited, it is difficult to form a solution containing a high concentration thereof (at most 25% by weight in terms of the nonvolatile matter content in a solution).
In some cases, the addition of carbon black to the polyamic acid solution sharply increases viscosity, which makes it difficult to grind the carbon black by the impulsive force generated between the balls in a dispersing apparatus such as a ball mill. In order to uniformly dispersing carbon black in a polyamic acid solution, pulverizability of carbon black with a disperser and the interfacial phenomenon referred to as “wetting” of carbon black disintegrated by a solvent are required. Therefore, a large amount of organic polar solvent is added to uniformly disperse the carbon black. As a result, however, the nonvolatile matter content in the obtained masterbatch solution containing a high concentration of carbon black is as low as 16% by weight or less.
Furthermore, a solution containing a low concentration of polyamic acid has drawbacks in that a thicker film is difficult to form in a single step, and in that since a larger amount of solvent is required, the period of time for removing the large amount of solvent by evaporation is prolonged.
Because the three steps described above are included in the polyimide film preparation method, the entire process requires considerable time and cost. Thus, there is room for improving efficiency and cost.
Japanese Unexamined Patent Publication No. 10-182820 discloses a film-formation method using a polyimide precursor composition comprising as a main component a monomer having a mixture of aromatic tetracarboxylic acid component comprising as a main component asymmetric aromatic tetracarboxylic acid and/or ester thereof (specifically 60 mol % or more of 2,3,3′4′-biphenyl tetracarboxylic acid and/or ester thereof) and an approximately equimolar amount of an aromatic diamine component. Moreover, Japanese Unexamined Patent Publication No. 10-182820 discloses a method for forming a polyimide film by applying a polyimide precursor composition to a glass plate by pouring the same thereon and then heating the result (raising the temperature stepwise in the range of 80° C. to 350° C.), and also discloses using the polyimide film as an electrically conductive paste by the addition of silver powder, copper powder, carbon black, etc.
However, a semi-conductive polyimide film obtained by the above-mentioned film formation method has further room for improvement in its properties, such as electrical resistance, etc., when it is used as an intermediate transfer belt or the like in an electrophotographic system for use in color printers, color copiers, etc., which are requiring high accuracy in recent years.